LCD

We’ve already learned how a flat panel LCD works in the section Learn about LCD Displays. Now it’s time to learn how the technology works in a projector.

An LCD optical light engine incorporates three small LCD panels (see Figure 1.): a lamp, filters and a prism to create an image.

Figure 1

White light is provided by way of a lamp. The lamp light is first passed through a polarizing filter, and is then passed through a series of dichroic mirrors (a mirror that reflects only certain wavelengths of light while letting all others pass through), which split the white light into its primary colors: red, green and blue. The three primary colors are then passed through three separate LCD panels and sent to a dichroic prism, which recombines the three primary colors. After the three colors are recombined, the light is passed through the main lens and out of the projector. See Figure 2.

Figure 2

Each LCD panel controls one of the three primary colors. For example, if we were to show an image of an open green field with a blue sky, the LCD panel that controls red would block all light from passing on to the dichroic prism, while the LCD panel that controls blue would only allow blue light through in the of sky. Meanwhile, the LCD panel for green would do the same for the areas of the green field.

How does the LCD panel itself work? The light is polarized first. Think of the polarization process as a filter that only accepts light waves that are all traveling on the same plane. In this way, if you were to put two polarized filters back-to-back that filtered the light in opposite planes, all the light would be blocked from passing through.

This polarized light travels to the LCD panel and passes through a pane of glass to the liquid crystal inside. Liquid crystals have the unique property of bending light as the light travels though. In this manner, the polarized light leaves the liquid crystal, actually traveling on a different plane that when it entered.

If you apply current to liquid crystal, the crystals align themselves and will not bend the light. Instead the light leaves the liquid crystal on the same plane that it entered.

If you align a second polarizing filter after the light passes through the liquid crystal, you can block all light from escaping if the light waves are traveling on a plane that the filter blocks. See Figure 3.

Figure 3

Each LCD panel has individual cells that are each controlled by a separate transistor that can apply current to the liquid crystal within the cell. The resolution of the display determines the number of cells, or pixels, that the LCD panel has.

In this way, each panel can control what color each pixel is going to be at any given moment. If a pixel needs to be white, all three panels let through light at their corresponding pixel and the three primary colors are recombined by the dichroic prism to create white light.

All pixels working together in the projector can then paint a picture in much the same way a printer uses individual dots to make a color image on paper.